Method and device for vibration control

ABSTRACT

A vibration control system comprising an actuator, and a sensor useful for controlling vibrations in systems for fabricating electronics equipment. The actuator may comprise one or more plates or elements of electroactive material bonded to an electroded sheet.

Related Applications

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/491,969, filed Jan. 27, 2000, which claims the benefit ofU.S. application Ser. No. 60/117,671, filed Jan. 28, 1999, and is acontinuation-in-part of U.S. application Ser. No. 09/261,475, filed Feb.26, 1999, which is a continuation-in-part of U.S. application Ser. No.08/943,645, filed Oct. 3, 1997, which is a continuation of U.S.application No. 08/188,145, filed Jan. 27, 1994, the disclosures of eachof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] In the competitive marketplace which exists for automatedsurface-mount (SMT) electronics equipment, including systems forfabricating electronics equipment or components, improvements inaccuracy and speed are a significant advantage. Such equipment is oftenused in fabricating, for example, semiconductor chips, printed circuitboards, liquid crystal displays, and thin film devices, and may featuremultiple gantry/head assemblies, linear motors, photoimaging systems,etching systems, and/or a number of other technologies. The presentinvention relates to devices and methods for reducing vibration inherentin such equipment during operation thereby to improve the speed and/oraccuracy of such equipment.

[0003] For example, modem photolithography tools require extremely highexposure accuracy. This can only be achieved if the levels of elasticdisplacement at crucial points in the tool do not exceed severalnano-meters. Since lithography tools contain numerous moving parts suchas the reticle and wafer stages, they are subject to persistentdisturbing forces acting on their structure. Moreover, the toolstructure is subject to environmental disturbances such as floorvibrations and air turbulence. While the level of these disturbances canbe reduced, they cannot be eliminated in their entirety.

[0004] There are a number of existing techniques employed to limit theelastic vibration of lithography tools. For example, the stiffness ofthe structure that supports key elements such as the lens assembly maybe increased, tuned mass dampers may be used, the signals applied to themoving stages may be shaped, or the floor vibrations may be isolatedusing actively controlled air springs. While effective in reducingelastic vibration, these methods often do not meet the stringentrequirements of more advanced photolithography tools.

[0005] Current efforts to control vibration on SMT placement equipmentinclude placing frictional damping device at the end of the gantry. This“friction block” serves mainly to stabilize the gantry and headtrajectory control system, but it also has been shown to reduce thesettling time during certain pick and place operations. However, theeffectiveness of the friction block depends on precise tuning of thenormal force (or pre-load). The friction block tends to wear outquickly, greatly reducing its effectiveness and contaminating the restof the machine with particles. Moreover, the friction block worksagainst rigid body movement, resulting in slower operation of theequipment. The vibration control system of the present invention, whichcomprises an actuator assembly, serves to replace the friction blockentirely while improving settling time, or, alternatively, to operate inconjunction with the friction block, providing additional accuracy orspeed of operation.

[0006] One aspect of the present invention relates to actuator elementsuseful for active vibration reduction, structural control, dynamictesting, precision positioning, motion sensing and control, and activedamping. Electroactive materials, such as piezoelectric,electrostrictive or magnetostrictive materials, are useful in suchtasks. In one embodiment of the invention, bare electroactive elementsare used. In another embodiment, packaged electroactive elements, asdescribed herein, are used.

[0007] Thus, improvements are desirable in the manner in which vibrationis controlled in systems for fabricating electronic components, as wellas the manner in which an actuator is attached to the equipment to becontrolled.

SUMMARY OF THE INVENTION

[0008] In one embodiment of the invention, a vibration control system isprovided comprising an actuator assembly, and a sensor for sensing aparameter of movement or performance. The vibration control system isparticularly useful for controlling vibration in systems for fabricatingelectronics components, which often include one or more gantryassemblies, head assemblies, and/or moving stages or components.Contemplated systems for fabricating electronics components include, butare not limited to, pick and place systems, lithography systems, andthose used to fabricate semiconductor chips, printed circuit boards,liquid crystal displays, and thin film devices. However, the devices andmethods of the invention would be useful in fabricating systems of anysort, such as machine tool equipment, milling equipment, or systems usedin an automated assembly line. Also contemplated are systems forfabricating electronic components wherein the systems comprise a lenssystem, a wafer stage, and a structure for supporting the lens systemand wafer stage where the lens system creates an image on the waferstage such as would be used in modern photolithography.

[0009] In one embodiment, an active vibration control system for usewith a photolithography fabricating system includes the followingcomponents: a sensor that measures the displacement levels at the keypoints, or provides information from which such information can beestimated; a digital or analog processor that can compute a controlsignal based on the sensors input, and an actuator that can induceelastic displacement in the structure.

[0010] In a particularly preferred embodiment, an actuator useful in anactive vibration control system used in conjunction withphotolithography tools is non-reactive and does not require back support(actuators that require back support may excite elastic vibrations inthe support structure, which may be re-introduced unto the tool), andhas a very low distortion profile (an actuator array designed to controlstructural vibration at a given frequency or band must not excite anyvibration outside that band).

[0011] In a particularly preferred embodiment, a vibration controlsystem in accordance with the invention comprises an induced-strainactuator that acts directly on the strain state of the structure, andhas virtually no distortion. Such an actuator can excite, and thereforecontrol, only the elastic vibration modes of the controlled structure,leaving all other vibration modes (such as the modes of variousequipment housing structures, etc.) uncontrolled. This contributes tothe control system simplicity and robustness.

[0012] In another preferred embodiment of the invention, the vibrationcontrol system further comprises a circuit in electrical communicationwith the actuator assembly and the sensor. In one embodiment, the sensorrelays information about movement, vibration or performance to thecircuit, which, in response, signals the actuator assembly to controlvibration. The vibration in the systems in which the present inventionare useful may be due to external disturbance or due to the inherentdisturbances generated by the system itself.

[0013] In yet another preferred embodiment of the invention, thevibration control system further comprises an electrical connection tothe fabricating system. The electrical connection may provide for thefabricating system to send to, or receive from the vibration controlsystem information such as abling or disabling signals, system statussignals, or fault/error status signals. In another embodiment, a circuitaccording to the invention further comprises a control system comprisingat least one controller. Such a control system may permit auto-tuning,gain scheduling, external gain control, or it may be a linear feedforward control, or may serve as another source of feedback control.

[0014] In an embodiment of the invention wherein the vibration controlsystem has an auto-tuning control, prior to operation, the controlsystem injects one or more test signals into the system and measures theresponse. The measured response is used to refine an internal model ofthe plant, and the control gains are modified accordingly. Control gainsare kept constant while the loop is closed.

[0015] In an embodiment of the invention wherein the vibration controlsystem has a gain scheduling control, the controllers are designed forthe system at several different operating points. In the case of a pickand place machine, these points would be different positions of the pickand place head. The controllers are stored in memory in the digitalcontrol system. During operation, sensors feed information to thecontroller describing the configuration of the machine in real time. Asthe system moves through each operating point, the control systemswitches to the optimal control gains for that point. A variant of thisis that the control gains used at any point in time are a linearinterpolation of the gains from several controllers stored in memory forseveral nearby operating points.

[0016] In an embodiment of the invention wherein the vibration controlsystem has an external gain control, the control system includes aninput which connects to the computer system which monitors the overallperformance of the machine. The controller implemented at any instant intime has a gain which is proportional to this signal. The monitoringsystem modifies this gain until optimal performance is achieved. Ifperformance begins to move out of specification due to slow timevariation, the monitoring system would repeat the gain optimizationsequence.

[0017] In an embodiment of the invention wherein the vibration controlsystem has a feed forward control, in addition to the feedback control(controller driven by signals originating from sensors which monitor thestructural vibration), an additional signal which is in phase with aharmonic disturbance (such as motor rotation) provided to thecontroller. The controller feeds forward a filtered version of thissignal. The gains which adjust the magnitude and phase of the feedforward control relative to the disturbance signal are adjustedadaptively to minimize the influence of the disturbance on theperformance.

[0018] In certain embodiments of the invention, the actuator assemblymay comprise a strain actuator, an electroactive strain actuator, apiezoceramic strain actuator, an electroactive stack actuator, or atleast two actuators. In yet another embodiment of the invention, theactuator assembly is in electrical communication with the sensor.

[0019] Also in certain embodiments of the invention, the sensor maycomprise a strain sensor, an accelerometer, laser displacement sensor,laser interferometer, or at least two sensors. In another embodiment ofthe invention, the sensor may comprise at least two sensors measuring atleast two different signals. In a preferred embodiment, the sensordirectly measures some aspect directly related to performance of thesystems in which the present invention is useful.

[0020] In a particularly preferred embodiment of the invention, thevibration control system comprises an electronic link or cable providinginformation about the trajectory of a gantry and head.

[0021] An actuator assembly according to the present invention mayinclude one or more strain elements, such as a piezoelectric orelectrostrictive plate, shell, fiber or composite; a housing forming aprotective body about the element; and electrical contacts mounted inthe housing and connecting to the strain element; these parts togetherforming a flexible card. At least one side of the assembly includes athin sheet which is attached to a major face of the strain element, andby bonding the outside of the sheet to an object a stiff shear-freecoupling is obtained between the object and the strain element in thehousing.

[0022] In a preferred embodiment, the strain elements are piezoceramicplates, which are quite thin, preferably between slightly under aneighth of a millimeter to several millimeters thick, and which have arelatively large surface area, with one or both of their width andlength dimensions being tens or hundreds of times greater than thethickness dimension. A metallized film makes electrode contact, while abonding agent and insulating material hermetically seal the deviceagainst delamination, cracking and environmental exposure. The bondingagent used may be an epoxy, such as B-stage or C-stage epoxy, athermoplastic, or any other material useful in bonding together thepiezoceramic plate, metallized film and insulating material. Thespecific bonding agent used will depend on the intended application ofthe device. In a preferred embodiment, the metallized film andinsulating material are both provided in a flexible circuit of toughpolymer material, which thus provides robust mechanical and electricalcoupling to the enclosed elements. Alternatively, the metallized filmmay be located directly on the piezoceramic plate, and the insulatingmaterial may have electrical contacts.

[0023] By way of illustration, an example below describes a constructionutilizing rectangular PZT plates a quarter millimeter thick, with lengthand width dimensions each of one to three centimeters, each element thushaving an active strain-generating face one to ten square centimeters inarea. The PZT plates are mounted on or between sheets of a stiff strongpolymer, e.g., one half, one or two mil polyimide, which is copper cladon one or both sides and has a suitable conductive electrode patternformed in the copper layer for contacting the PZT plates. Variousspacers surround the plates, and the entire structure is bonded togetherwith a structural polymer into a waterproof, insulated closed package,having a thickness about the same as the plate thickness, e.g., 0.30 to0.50 millimeters. So enclosed, the package may bend, extend and flex,and undergo sharp impacts, without fracturing the fragile PZT elementswhich are contained within. Further, because the conductor pattern isfirmly attached to the polyimide sheet, even cracking of the PZT elementdoes not sever the electrodes, or prevent actuation over the full areaof the element, or otherwise significantly degrade its performance.

[0024] The thin package forms a complete modular unit, in the form of asmall “card”, complete with electrodes. The package may thenconveniently be attached by bonding one face to a structure so that itcouples strain between the enclosed strain element and the structure.This may be done for example, by simply attaching the package with anadhesive to establish a thin, high shear strength, coupling with the PZTplates, while adding minimal mass to the system as a whole. The platesmay be actuators, which couple energy into the attached structure, orsensors which respond to strain coupled from the attached structure.

[0025] In different embodiments, particular electrode patterns areselectively formed on the sheet to either pole the PZT plates in-planeor cross-plane, and multiple layers of PZT elements may be arranged orstacked in a single card to result in bending or shear, and evenspecialized torsional actuation.

[0026] In accordance with a further aspect of the invention, circuitelements are formed in, or with, the vibration control system to filter,shunt, or process the signal produced by the PZT elements, to sense themechanical environment, or even to locally perform switching or poweramplification for driving the actuation elements. The actuator packagemay be formed with pre-shaped PZT elements, such as half-cylinders, intomodular surface-mount shells suitable for attaching about a pipe, rod orshaft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other desirable properties of the invention will beunderstood from the detailed description of illustrative embodiments,wherein:

[0028]FIG. 1 A is a system illustration of a typical prior art actuator;

[0029]FIGS. 1B and 1C are corresponding illustrations of two systems inaccordance with the present invention;

[0030]FIGS. 2A and 2B show top and cross-sectional views, respectively,of a basic actuator or sensor card in accordance with the presentinvention; FIG. 2C illustrates an actuator or sensor card with circuitelements;

[0031]FIG. 3 illustrates another card;

[0032]FIGS. 4A and 4B show sections through the card of FIG. 3;

[0033]FIGS. 5 and 5A show details of the layer structure of the card ofFIG. 3;

[0034]FIG. 6 shows an actuator package comb electrodes for in-planeactuation;

[0035]FIG. 7 illustrates a torsional actuator package using the cards ofFIG. 6;

[0036]FIGS. 8A and 8B show actuators mounted as surface mount actuatorson a surface or rod, respectively;

[0037]FIG. 9 shows actuators mounted as mechanical elements;

[0038]FIG. 10 shows a block diagram of an embodiment of an electroactivevibration control system for a gantry;

[0039]FIG. 11 shows a simulated frequency response on a collect andplace head at the tip of a gantry, without and with electroactivevibration control;

[0040]FIG. 12 shows the simulated time response of a collect and placehead without and with electroactive control;

[0041]FIG. 13 shows extensional strain energy concentration;

[0042]FIG. 14 shows the results of a closed loop test on the frequencyresponse of a pick and place machine having a vibration control systemin accordance with the invention;

[0043]FIG. 15 shows the results of a closed loop test on the gaincontrol of a pick and place machine having a vibration control system inaccordance with the invention;

[0044]FIG. 16 shows the power spectral density of error signals recordedby a laser metrology system in a lithography machine;

[0045] FIGS. 17-20 show different embodiments of the invention as usedwith a fabricating system;

[0046]FIG. 21 shows an embodiment of the invention as used with afabricating system;

[0047]FIG. 22 shows a simplified two-dimensional physics model of awafer stage and wafer stage base;

[0048]FIG. 23 shows controller results;

[0049]FIG. 24 shows a block diagram for stage control;

[0050]FIG. 25 shows experimental and analytical results;

[0051]FIG. 26 shows experimental and analytical results.

DETAILED DESCRIPTION OF THE INVENTION

[0052] Applicants have developed a vibration control system particularlyuseful for controlling vibration in a system for fabricating electronicscomponents. The vibration control system of the invention is useful forcontrolling vibration that is either externally produced in the systemfor fabricating components, or is internal to or inherent in the system.Internal vibration may be caused by various motors, such as step or D.C.motors, or hydraulic or pneumatic actuators used in a fabricatingsystem.

[0053] A vibration control system according to the invention maycomprise electroactive actuators and sensors, integrated with thefabricating system. The control and power electronics may be separateunits, located adjacent to the equipment and connected to the actuatorsand sensors through appropriate linking cabling. Alternatively, thecontrol and power electronics may be a fully integrated system with thefabricating system.

[0054] The electroactive actuator may be secured to or within thefabricating system in various ways. As shown in FIGS. 17, 19, and 20,for example, the actuator may be fixed into place by a bolt 414 eitherpushing against or going through the actuator. Alternatively, theactuator may be secured by friction, tension, or otherwise force fit. Inone embodiment, as shown in FIG. 18, the actuator is bonded to a plate412, which, in turn, is bolted to a component of a the fabricatingsystem with bolts 414, 414′, 414″, and 414′″. In another embodiment, theactuator is bonded to a plate, which is bolted to a second plate, andthe second plate is then bolted to a component of the fabricatingcomponent. In another embodiment, the actuator assembly is detachablysecured within the vibration control system, or detachably secured to acomponent of a fabricating system.

[0055]FIG. 21 shows an embodiment of the invention as used in afabricating system. In this embodiment, the fabricating system comprisesa wafer stage 400, a reticle stage 402, laser interferometers 404, 404′,404″, and 404′″ with X&Y mirrors, and a support structure 406. Thesupport structure 406 supports a lens assembly 410. The interferometers404, 404′, 404″, and 404′″ are located on the wafer stage 400, thereticle stage 402, and on the lens assembly 410. Mounted on the supportstructure 406 are two actuators 408 and 408′ comprising, for example, anelectroactive element. Each of the actuators 408 and 408′ are inelectrical communication with a circuit. Signals from theinterferometers 404, 404′, 404″, and 404′″ are relayed through an SBCanalog I/O channel and amplifiers to the actuators 408 and 408′, which,in response, controls vibration within the fabricating system. Bycontrolling the vibration within the fabricating system, the accuracy ofthe placement and absolute size of the metallized traces in thesemiconductor on a wafer stage may be improved. Alternatively or inaddition, the through-put of the fabrication system may be increasedwithout decreasing accuracy.

[0056] Useful in this invention are electroactive actuator assemblies.FIG. 1 A illustrates in schema the process and overall arrangement of aprior art surface mounted piezoelectric actuator assembly 10. Astructure 20, which may be a structural or machine element, a plate,airfoil or other interactive sheet, or a device or part thereof has asheet 12 of smart material bonded thereto by some combination ofconductive and structural polymers, 14, 16. An insulator 18, which maybe formed entirely or in part of the structural polymer 16, encloses andprotects the smart material, while conductive leads or surfaceelectrodes are formed or attached by the conductive polymer. An externalcontrol system 30 provides drive signals along lines 32 a, 32 b to thesmart material, and may receive measurement signals from surface-mountedinstrumentation such as a strain gauge 35, from which it derivesappropriate drive signals. Various forms of control are possible. Forexample, the strain gauge may be positioned to sense the excitation of anatural resonance, and the control system 30 may simply actuate the PZTelement in response to a sensor output, so as to stiffen the structure,and thereby shift its resonant frequency. Alternatively, a vibrationsensed by the sensor may be fed back as a processed phase-delayeddriving signal to null out an evolving dynamic state, or the actuatormay be driven for motion control. In better-understood mechanicalsystems, the controller may be programmed to recognize empiricalconditions, i.e., aerodynamic states or events, and to select specialcontrol laws that specify the gain and phase of a driving signal foreach actuator 12 to achieve a desired change.

[0057] For all such applications, major work is required to attach thebare PZT plate to its control circuitry and to the workpiece, and manyof the assembly steps are subject to failure or, when quantitativecontrol is desired, may require extensive modeling of the device afterit has been assembled, in order to establish control parameters for auseful mode of operation that are appropriate for the specificthicknesses and mechanical stiffness achieved in the fabricationprocess. A benefit of packaging an electroactive element when bonding tothe plate is that electrical isolation or capacitive decoupling from theplate, structure or any part of the fabrication system may be achieved.

[0058]FIG. 1B shows an actuator assembly useful in one embodiment of thepresent invention. As shown, it is a modular pack or card 40 that simplyattaches to a structure 20 with a quick setting adhesive, such as afive-minute epoxy 13, or in other configurations attaches at a point orline. The operations of sensing and control thus benefit from a morereadily installable and uniformly modeled actuator structure. Inparticular, the modular pack 40 has the form of a card, a stiff butbendable plate, with one or more electrical connectors preferably in theform of pads located at its edge (not shown) to plug into a multi-pinsocket so that it may connect to a simplified control system 50. Asdiscussed in greater detail below with respect to FIG. 2C, the modularpackage 40 may also incorporate planar or low-profile circuit elements,which may include signal processing elements, such as weighting orshunting resistors, impedance matchers, filters and signal conditioningpreamplifiers, and may further include switching transistors and otherelements to operate under direct digital control, so that the onlyexternal electrical connections necessary are those of a microprocessoror logic controller, and a power supply.

[0059] In a further embodiment particularly applicable to some low powercontrol situations, a modular package 60 as shown in FIG. 1C may includeits own power source, such as a battery or power cell, and may include acontroller, such as a microprocessor chip or programmable logic array,to operate on-board drivers and shunts, thus effecting a complete set ofsensing and control operations without any external circuit connections.

[0060] The present invention specifically pertains to piezoelectricpolymers, and to materials such as sintered metal zirconate, niobatecrystal or similar piezoceramic materials that are stiff, yet happen tobe quite brittle. It also pertains to electrostrictive materials. Asused in the claims below, both piezoelectric and electrostrictiveelements, in which the material of the elements has an electromechanicalproperty, will be referred to as electroactive elements. High stiffnessis essential for efficiently transferring strain across the surface ofthe element to an outside structure or workpiece, typically made ofmetal or a hard structural polymer, and the invention in its actuatoraspect does not generally contemplate soft polymer piezoelectricmaterials. While the terms “stiff” and “soft” are relative, it will beunderstood that in this context, the stiffness, as applied to anactuator, is approximately that of a metal, cured epoxy, high-techcomposite, or other stiff material, with a Young's modulus greater than0.1×10^(6,) and preferably greater than 0.2×10^(6.) When constructingsensors, instead of actuators, the invention also contemplates the useof low-stiffness piezoelectric materials, such as polyvinylidenedifluoride (PVDF) film and the substitution of lower cure temperaturebonding or adhesive materials. The principal construction challenges,however, arise with the first class of piezo material noted above, andthese will now be described.

[0061] In general, the invention includes novel forms of actuators andmethods of making such actuators, where “actuator” is understood to meana complete and mechanically useful device which, when powered, couplesforce, motion or the like to an object or structure. In its broad form,the making of an actuator involves “packaging” a raw electroactiveelement to make it mechanically useful. By way of example, rawelectroactive piezoelectric materials or “elements” are commonlyavailable in a variety of semi-processed bulk material forms, includingraw piezoelectric material in basic shapes, such as sheets, rings,washers, cylinders and plates, as well as more complex or compositeforms, such as stacks, or hybrid forms that include a bulk material witha mechanical element, such as a lever. These materials or raw elementsmay have metal coated on one or more surfaces to act as electricalcontacts, or may be non-metallized. In the discussion below,piezoelectric materials shall be discussed by way of example, and allthese forms of raw materials shall be referred to as “elements”,“materials”, or “electroactive elements”. As noted above, the inventionfurther includes structures or devices made by these methods andoperating as transducers to sense, rather than actuate, a strain,vibration, position or other physical characteristic, so that whereapplicable below, the term “actuator” may include sensing transducers.

[0062] Embodiments of the invention employ these stiffelectrically-actuated materials in thin sheets—discs, annuli, plates andcylinders or shells—that are below several millimeters in thickness, andillustratively about one fifth to one quarter millimeter thick.Advantageously, this thin dimension allows the achievement of highelectric field strengths across a distance comparable to the thicknessdimension of the plate at a relatively low overall potential difference,so that full scale piezoelectric actuation may be obtained with drivingvoltages of ten to fifty volts, or less. Such a thin dimension alsoallows the element to be attached to an object without greatly changingthe structural or physical response characteristics of the object.However, in the prior art, such thin elements are fragile, and may breakdue to irregular stresses when handled, assembled or cured. The impactfrom falling even a few centimeters may fracture a piezoceramic plate,and only extremely small bending deflections are tolerated beforebreaking.

[0063] In accordance with the present invention, the thin electricallyactuated element is encased by layers of stiff insulating material, atleast one of which is a tough film which has patterned conductors on oneof its surfaces, and is thinner than the element itself. A package isassembled from the piezo elements, insulating layers, and variousspacers or structural fill material, such that altogether theelectrodes, piezo element(s), and enclosing films or layers form asealed card of a thickness not substantially greater than that of thebare actuating element. Where elements are placed in several layers, aswill be described below, the package thickness is not appreciablygreater than the sum of the thicknesses of the stacked actuatingelements.

[0064]FIG. 2A illustrates a basic embodiment 100 of the invention. Athin film 110 of a highly insulating material, such as a polyimidematerial, is metallized, typically copper clad, on at least one side,and forms a rectangle which is coextensive with or slightly larger thanthe finished actuator package. A suitable material available for use infabricating multilayer circuit boards is distributed by the RogersCorporation of Chandler, Ariz. as their Flex-I-Mid 3000 adhesivelesscircuit material, and consists of a polyimide film formed on a rolledcopper foil. A range of sizes are available commercially, with the metalfoils being of 18 to 70 micrometer thickness, integrally coated with apolyimide film of 13 to 50 micrometer thickness. Other thicknesses maybe fabricated. In this commercial material, the foil and polymer filmare directly attached without adhesives, so the metal layer may bepatterned by conventional masking and etching, and multiple patternedlayers may be built up into a multilayer board in a manner describedmore fully below, without residual adhesive weakening the assembly orcausing delamination. The rolled copper foil provides high in-planetensile strength, while the polyimide film presents a strong, tough anddefect-free electrically insulating barrier.

[0065] In constructions described below, the film constitutes not onlyan insulator over the electrodes, but also an outer surface of thedevice. It is therefore required to have high dielectric strength, highshear strength, water resistance and an ability to bond to othersurfaces. High thermal resistance is necessary in view of thetemperature cure used in the preferred fabrication process, and is alsorequired for some application environments. In general, polyamide/imideshave been found useful, but other materials, such as polyesters orthermoplastics with similar properties, may also be used.

[0066] In the present constructions, the foil layer is patterned byconventional masking and etch techniques (for example, photoresistmasking and patterning, followed by a ferric chloride etch), to formelectrodes for contacting the surface of piezo plate elements.Alternatively, a more ductile, thin conductive layer may be used. Forexample, a thin conductive layer may be printed on the polymer film ordirectly on the piezoelectric element using silver conductive ink. InFIG. 2A, electrodes 111 extend over one or more sub-regions of theinterior of the rectangle, and lead to reinforced pads or lands 111 a,111 b extending at the edge of the device. The electrodes are arrangedin a pattern to contact a piezoelectric element along a broadly-turningpath, which crosses the full length and width of the element, and thusassures that the element remains connected despite the occurrence of afew cracks or local breaks in the electrode or the piezo element. Framemembers 120 are positioned about the perimeter of sheet 110, and atleast one piezoelectric plate element 112 is situated in the centralregion so that it is contacted by the electrodes 111. The frame membersserve as edge binding, so that the thin laminations do not extend to theedge, and they also function as thickness spacers for the hot-pressassembly operation described further below, and as position-markerswhich define the location of piezo plates that are inserted during theinitial stages of assembling the laminated package.

[0067]FIG. 2A is a somewhat schematic view, inasmuch as it does not showthe layer structure of the device which secures it together, including afurther semi-transparent top layer 116 (FIG. 2B), which in practiceextends over the plate 112 and together with the spacers 120 and sheet110 closes the assembly. A similar layer 114 is placed under the piezoelement, with suitable cut-outs to allow the electrodes 111 to contactthe element. Layers 114, 116 are preferably formed of a curable epoxysheet material, which has a cured thickness equal to the thickness ofthe metal electrode layer, and which acts as an adhesive layer to jointogether the material contacting it on each side. When cured, this epoxyconstitutes the structural body of the device, and stiffens theassembly, extending entirely over a substantial portion of the surfaceof the piezo element to strengthen the element and arrest crack growth,thereby enhancing its longevity. Furthermore, epoxy from this layeractually spreads in a microscopically thin but highly discontinuousfilm, about 0.0025 mm thick, over the electrodes, bonding them firmly tothe piezo plate, but with a sufficient number of voids and pinholes sothat direct electrical contact between the electrodes and piezo elementsstill occurs over a substantial and distributed contact area.

[0068]FIG. 2B shows a cross-sectional view, not drawn to scale, of theembodiment of FIG. 2A. By way of rough proportions, taking thepiezoelectric plate 112 as 0.2-0.25 millimeters in thickness, theinsulating film 110 is much thinner, no more than one-tenth to one-fifththe plate thickness, and the conductive copper electrode layer 111 mayhave a thickness typically of ten to fifty microns, although the latterrange is not a set of strict limits, but represents a useful range ofelectrode thicknesses that are electrically serviceable, convenient tomanufacture and not so thick as to either impair the efficiency ofstrain transfer or introduce delamination problems. The structural epoxy114 fills the spaces between electrodes 111 in each layer, and hasapproximately the same thickness as those electrodes, so that the entireassembly forms a solid block. The spacers 120 are formed of a relativelycompressible material, having a low modulus of elasticity, such as arelatively uncrosslinked polymer, and, when used with a pressure-curedepoxy as described below, are preferably of a thickness roughlyequivalent to the piezoceramic plate or stack of elements, so that theyform an edge binding about the other components between the top andbottom layers of film 110.

[0069] A preferred method of manufacture involves applying pressure tothe entire package as the layer 116 cures. The spacers 120 serve toalign the piezoceramic plates and any circuit elements, as describedbelow with reference to FIGS. 3-5, and they form a frame that iscompressed slightly during assembly in the cure step, at which time itmay deform to seal the edges without leaving any stress orirregularities. Compression eliminates voids and provides a dense andcrack-free solid medium, while the curing heat effects a high degree ofcross-linking, resulting in high strength and stiffness.

[0070] An assembly process for the embodiment of FIGS. 2A, 2B is asfollows. One or more pieces of copper clad polyimide film, eachapproximately 0.025 to 0.050 millimeters thick in total, are cut to asize slightly larger than the ultimate actuator package dimensions. Thecopper side of the film is masked and patterned to form the desiredshape of electrodes for contacting a piezo element together withconductive leads and any desired lands or access terminals. A pitchforkelectrode pattern is shown, having three tines which are positioned tocontact the center and both sides of one face of a piezo element, but inother embodiments an H- or a comb-shape is used. The patterning may bedone by masking, etching and then cleaning, as is familiar from circuitboard or semiconductor processing technology. The masking is effected byphotoresist patterning, screening, tape masking, or other suitableprocess. Each of these electroded pieces of polyimide film, like aclassical printed circuit board, defines the positions of circuitelements or actuator sheets, and will be referred to below simply as a“flex circuit.” However, methods and devices of the invention alsocontemplate using an electroded piezo element, an insulator, andelectrical contacts, rather than a “flex circuit”.

[0071] Next, uncured sheet epoxy material having approximately the samethickness or slightly thicker than the electrode foil layer is cut,optionally with through-apertures matching the electrode pattern toallow enhanced electrical contact when assembled, and is placed overeach flex circuit, so it adheres to the flex circuit and forms aplanarizing layer between and around the electroded portions. Thebacking is then removed from the epoxy layers attached to the flexcircuits, and pre-cut spacers 120 are placed in position at comer andedges of the flex circuit. The spacers outline a frame which extendsabove the plane of the electrodes, and defines one or more recesses intowhich the piezo elements are to be fitted in subsequent assembly steps.The piezo element or elements are then placed in the recesses defined bythe spacers, and a second electroded film 111, 112 with its ownplanarizing/bonding layer 114 is placed over the element in a positionto form electrode contacts for the top of the piezo element. If thedevice is to have several layers of piezo elements, as would be the casefor some bending actuator constructions, these assembly steps arerepeated for each additional electroded film and piezoelectric plate,bearing in mind that a polyimide film which is clad and patterned onboth sides may be used when forming an intermediate electrode layer thatis to contact actuator elements both above and below the intermediatesheet.

[0072] Once all elements are in place, the completed sandwich assemblyof patterned flex circuits, piezo sheets, spacers and curable patternedepoxy layers is placed in a press between heated platens, and is curedat an elevated temperature and pressure to harden the assembly into astiff, crack-free actuator card. In a representative embodiment, a curecycle of thirty minutes at 350° F. and 50-100 psi pressure is used. Theepoxy is selected to have a curing temperature below the depolingtemperature of the piezo elements, yet achieve a high degree ofstiffness.

[0073] The above construction illustrates a simple actuator card havinga single piezo plate sandwiched between two electroded films, so thatthe plate transfers shear strain efficiently through a thin film to thesurface of the actuator card. The measure of transfer efficiency, givenby the shear modulus divided by layer thickness squared, and referred toas gamma (Γ), depends on the moduli and thickness of the epoxy 114, therolled foil electrodes 111, and the polyimide film 110. In arepresentative embodiment in which the epoxy and copper electrode layersare 1.4 mils thick and the epoxy has a modulus of 0.5×10⁶, a gamma ofapproximately 9×10¹⁰ pounds/inch⁴ is achieved. Using a thinner epoxylayer and film with 0.8 mil foil, substantially higher Γ is achieved. Ingeneral, the gamma of the electrode/epoxy layer is greater than 5×10¹⁰pounds/inch⁴, while that of the film is greater than 2×10¹⁰pounds/inch⁴.

[0074] It should be noted that using PZT actuator plates ten mils thick,a card having two PZT plates stacked over each other with three flexcircuit electroded film layers (the middle one being double clad tocontact both plates) has a total thickness of 28 mils, only fortypercent greater than the plates alone. In terms of mass loading, theweight of the actuator elements represents 90% of the total weight ofthis assembly. Generally, the plates occupy fifty to seventy percent ofthe package thickness, and constitute seventy to ninety percent of itsmass, in other constructions. Thus, the actuator itself allowsnear-theoretical performance modeling. This construction offers a highdegree of versatility as well, for implementing benders (as justdescribed) as well as stacks or arrays of single sheets.

[0075] Another useful performance index of the actuator constructed inaccordance with the present invention is the high ratio of actuatorstrain ε to the free piezo element strain Λ, which is approximately(0.8) for the two layer embodiment described herein, and in general isgreater than (0.5). Similarly, the ratio of package to free elementcurvatures, K, is approximately 0.85-0.90 for the describedconstructions, and in general is greater than 0.7.

[0076] Thus, overall, the packaging involved in constructing a piezoelement embedded in a flex circuit impairs its weight andelectromechanical operating characteristics by well under 50%, and aslittle as 10%, while profoundly enhancing its hardiness and mechanicaloperating range in other important respects. For example, while theaddition of sheet packaging structure to the base element would appearto decrease attainable K, in practical use the flex card constructionresults in piezo bender constructions wherein much greater totaldeflection may be achieved, since large plate structures may befabricated and high curvature may be repeatedly actuated, without crackfailure or other mechanical failure modes arising. Several Figures willillustrate the variety of constructions to which such enhanced physicalcharacteristics are brought.

[0077] First, the structure of an electroactive element embedded betweenflex circuits not only provides a low mass unified mechanical structurewith predictable response characteristics, but also allows theincorporation of circuit elements into or onto the actuator card. FIG.2C shows a top view of one device 70 of this type, wherein regions 71,73 each contain broad electroactive sheets, while a central region 72contains circuit or power elements, including a battery 75, a planarpower amplification or set of amplifiers 77, a microprocessor 79, and aplurality of strain gauges 78. Other circuit elements 82 a, 82 b may belocated elsewhere along the path of circuit conductors 81 about theperiphery. As with the other embodiments, spacers 120 define layout andseal edges of the device, while electrodes 111 attach the electroactiveelements to the processing or control circuitry which is now built-in.The circuit elements 82 a, 82 b may comprise weighting resistors if thedevice is operated as a sensor, or shunting resistors to implementpassive damping control. Alternatively, they may be filtering,amplifying, impedance matching or storage elements, such as capacitors,amplifiers or the like. In any case, these elements also are locatedaway from electroactive plates 84. The components collectively may sensestrain and implement various patterns of actuation in response to sensedconditions, or perform other sensing or control tasks.

[0078] Returning now to the actuator aspect of the invention, FIG. 3shows a top view of an actuator package 200 having dimensions of about1.25×9.00×030 inches and assembled with two layers of piezoelectricplates of four plates each. A rectangular polyimide sheet 210 with anend tab 210 a carries an electrode 211 in the form of a lattice ofH-shaped thin copper lines interconnected to each other and to a singlerunner 211 a that leads out to the tab, thus providing a low impedanceconnection directly to each of four rectangular regions which hold thepiezo plates.

[0079] Spacer elements 220 a, 220 b of H-shape, or 220 c of L-shape markoff comers and delineate the rectangular spaces for location of thepiezo plates 216. In this embodiment, a plurality of gaps 230, discussedfurther below, appear between adjacent the H- or L-spacers. As will beapparent from the description below, the use of these small discretespacer elements (I- , T- or O-shaped spacers may also be convenient) isenhanced because they may be readily placed on the tacky bonding epoxylayer 114 during assembly to mark out assembly positions and form areceiving recess for the piezo elements. However, the spacer structureis not limited to such a collection of discrete elements, but may be asingle or couple of frame pieces, formed as a punched-out sheet ormolded frame, to provide all, or one or more, orienting and/or sealingedges, or recesses for holding actuation of circuit components.

[0080]FIG. 5 illustrates a top view of each of the three sheet,electrode and piezo plate layers separately, while FIG. 5A illustratesthe general layering sequence of the film, conductor, and spacer/piezolayers. As shown, the spacers 220 and piezo plates 216 constitute asingle layer between each pair of electrode layers.

[0081]FIGS. 4A and 4B (not drawn to scale) illustrate the layerstructure of the assembled actuator along the vertical sections at thepositions indicated by “A” and “B” in FIG. 3. As more clearly shown inFIG. 4A, a patterned bonding layer of epoxy sheet 214 is coplanar witheach electrode layer 211 and fills the space between electrodes, whilethe spacer 220 c is coplanar with the piezo plate 216 and substantiallythe same thickness as the plate or slightly thicker. Illustratively, thepiezo plate 216 is a PZT-5A ceramic plate, available commercially in afive to twenty mil thickness, and has a continuous conductive layer 216a covering each face for contacting the electrodes 211. The spacers 220are formed of somewhat compressible plastic with a softening temperatureof about 250° C. This allows a fair degree of conformability at the curetemperature so the spacer material may fill slight voids 214 a (FIG. 4A)during the assembly process. As shown in FIG. 4B, the gaps 230 (whenprovided) between spacers result in openings 214 b which vent excessepoxy from the curable bonding layers 214, and fill with epoxy duringthe cure process. As illustrated in that FIGURE, a certain amount ofepoxy also bleeds over into patches of film 215 between the electrodes211 and the piezo plate 216. Because of the large and continuous extentof electrode 211, this patchy leakage of epoxy does not impair theelectrical contact with the piezo elements, and the additionalstructural connection it provides helps prevent electrode delamination.

[0082] It will be appreciated that with the illustrated arrangements ofelectrodes, each vertically stacked pair of piezo plates may be actuatedin opposition to each other to induce bending, or more numerous separateelectrodes may be provided to allow different pairs of plates to beactuated in different ways. In general, as noted above, the inventioncontemplates even quite complex systems involving many separate elementsactuated in different ways, with sensing, control, and power or dampingelements all mounted on the same card. In this regard, great flexibilityin adapting the card to practical tasks is further provided by itsflexibility. In general, it has a supple flexibility comparable to thatof an epoxy strip thirty mils thick, so that it may be bent, struck orvibrated without damage. It may also be sharply bent or curved in theregion of its center line CL (FIG. 3) where no piezo elements areencased, to conform to an attaching surface or comer. The elements maybe poled to change dimension in-plane or cross-plane, and the actuatorsmay therefore be attached to transmit strain to an adjacent surface in amanner effective to perform any of the above-described control actions,or to launch particular waveforms or types of acoustic energy, such asflexural, shear or compressional waves into an adjacent surface.

[0083]FIG. 6 shows another actuator embodiment 300. In this embodiment,illustrated schematically, the epoxy bonding layer, film and spacerelements are not shown, but only electrode and piezo sheets areillustrated to convey the operative mechanisms. A first set ofelectrodes 340 and second set 342 are both provided in the same layer,each having the shape of a comb with the two combs interdigitated sothat an electrical actuation field is set up between the tooth of onecomb and an adjacent tooth of the other comb. In FIG. 6, a parallel pairof combs 340 a, 342 a is provided on the other side of the piezo sheet,with comb electrode 340 tied to 340 a, and comb electrode 342 tied to342 a, so as to set up an electric field with equipotential lines “e”extending through the piezo sheet, and in-plane potential gradientbetween each pair of teeth from different combs. In the embodimentshown, the piezoceramic plates are not metallized, so direct electricalcontact is made between each comb and the plate. The plates are poledin-plane, by initially applying a high voltage across the combs tocreate a field strength above one two thousand volts per inch directedalong the in-plane direction. This orients the piezo structure so thatsubsequent application of a potential difference across the two-combelectrodes results in in-plane (shear) actuation. Thus, the directcontact of interdigital electrodes provides to the piezo element anelectrical field which is generally parallel to the actuation direction.

[0084] In addition to shear actuation, directional actuation and dampingmay be effected using methods or devices of the invention. For example,as shown in FIG. 7, two such actuators 300 may be crossed to providetorsional actuation.

[0085] In discussing the embodiments above, the direct transfer ofstrain energy through the electrode/polyimide layer to any adjoiningstructure has been identified as a distinct and novel advantage. Suchoperation may be useful for actuation tasks or diverse as airfoil shapecontrol actuation and noise or vibration cancellation or control. FIGS.8A and 8B illustrates typical installations of flat (FIG. 8A) andhemicylindrical (FIG. 8B) embodiments 60 of the actuator, applied to aflat or slightly curved surface, and a shaft, respectively.

[0086] However, while the electromechanical materials of these actuatorsoperate by strain energy conversion, applications of the presentinvention extend beyond strain-coupling through the actuator surface,and include numerous specialized mechanical constructions in which themotion, torque or force applied by the actuator as a whole is utilized.In each of these embodiments, the basic strip- or shell-shaped sealedactuator is employed as a robust, springy mechanical element, pinned orconnected at one or more points along its length. As shown in FIG. 9,when electrically actuated, the strip then functions, alone or withother elements, as a self-moving lever, flap, leaf spring, stack orbellows. In the diagrams of FIGS. 9(a)-9(q), the elements A, A′, A″ . .. are strip or sheet actuators such as shown in the above FIGURES, whilesmall triangles indicate fixed or pinned positions which correspond, forexample, to rigid mounting points or points of connection to astructure. Arrows indicate a direction of movement or actuation or thecontact point for such actuation, while L indicates a lever attached tothe actuator and S indicates a stack element or actuator.

[0087] The configurations of FIGS. 9(a)-9(c) as stacks, benders, orpinned benders may replace many conventional actuators. For example, acantilevered beam may carry a stylus to provide highly controlledsingle-axis displacement to constitute a highly linear, largedisplacement positioning mechanism of a pen plotter. Especiallyinteresting mechanical properties and actuation characteristics areexpected from multi-element configurations 9(d) et seq., whichcapitalize on the actuators having a sheet extent and being mechanicallyrobust. Thus, as shown in FIGS. 9(d) and (e), a pin-pin bellowsconfiguration may be useful for extended and precise one-axis Z-movementpositioning, by simple face-contacting movement, for applications suchas camera focusing; or may be useful for implementing a peristalsis-typepump by utilizing the movement of the entire face bearing against afluid. As noted in connection with FIG. 3, the flex circuit is highlycompliant, so hinged or folded edges may be implemented by simplyfolding along positions such as the centerline in FIG. 3, allowing aclosed bellows assembly to be made with small number of large,multi-element actuator units. The flex circuit construction allowsstrips or checkerboards of actuator elements to be laid out with foldlines between each adjacent pair of elements, and the fold lines may beimpressed with a thin profile by using a contoured (e.g. waffle-iron)press platen during the cure stage. With such a construction, an entireseamless bellows or other folded actuator may be made from a single flexcircuit assembly.

[0088] As noted above, the piezo element need not be a stiff ceramicelement, and if the flex circuit is to be used only as a sensor, theneither a ceramic element, or a soft material such as PVDF may beemployed. In the case of the polymer, a thinner more pliant lowtemperature adhesive is used for coupling the element, rather than ahard curable epoxy bonding layer.

[0089] Certain embodiments of the invention are exemplified below.

EXAMPLE 1

[0090] In this example, a vibration control system was designed todetermine certain parameters of functional requirements of a gantryactive control system. The functional requirements defined included (butwere not limited to) the following:

[0091] Accuracy

[0092] Settling time

[0093] Mass, size and location of the actuators and sensors

[0094] Power

[0095] Peak strains

[0096] Lifetime

[0097] Temperature range

[0098] Exposure to humidity and solvents

[0099] Cost

[0100] Interfaces with existing gantry control system

[0101] In order to gather data on the structural response of a gantryduring operation, the gantry was equipped with an array of piezoelectricstrain sensors and accelerometers. Placement and sizing of thepiezoelectric actuators required accurate strain mode shape information,which were obtained from this data, and were compared to the FiniteElement Model (“FEM”). One important piece of information obtained inthis phase of the project involved the effect of different headpositions on the dynamics. Both the actuator design and any controlsoftware design depended on when the vibration control was applied,i.e., while the head was moving along the gantry, and/or after it hadstopped at an arbitrary position on the gantry.

[0102] Data was acquired both with and without a friction block inplace, to allow at least analytical evaluation of the potential forcomplete replacement of the friction block by the electroactivevibration control system.

[0103] Using the data acquired above, along with finite element modelinginformation, the system-level design was performed. This design involvedselecting a system architecture, including actuator placement, type ofsensor, and the type of control algorithm. As discussed above, with themoving head having a significant effect on the gantry dynamics, theelectroactive vibration control system's effectiveness was improved bymaking the trajectory information available in the motion controlsystem. This information may be relayed to the motion control systemwith a simple clip lead attached to the proper point in the motioncontroller's circuitry. For example, information such as the plots ofmotor current, which is often easily accessible, may be provided to thevibration control system.

[0104] After selecting the system architecture, an analytical“input/output” model of the system was developed, to design the controlalgorithm for vibration control, and to simulate its performance. Thesystem design was compared to the functional requirements, to ensurecompliance. This analysis served to define the specifications on thevarious components of the control system, especially the analog sensorsignal conditioning electronics, the digital signal processor (DSP)based control unit, and the power amplifier used to provide thenecessary voltage and current to the electroactive actuators.

[0105] Each of the components of the electroactive vibration controlsystem were then designed, including the various electronic components.The electroactive actuators themselves were fabricated using methodsdisclosed herein. Each actuator was tested using standard qualitycontrol methods. All electronics were fabricated and tested forfunctionality and for compliance with the specifications devised in thesystem design task.

[0106] An important aspect of the design involved the integration of theactuators and sensors with the gantry. For example, for a given gantry,a 0.5 mm actuator thickness may be determined to not likely interferewith motion of the head along the gantry. The types of cable used toconnect the actuators and sensor on the gantry to the electronicequipment were then determined.

[0107] In this particular example, the gantry of an automated SMTelectronics collect and place equipment was equipped with actuators,sensors and electronics, and analyzed using an FEM with plate elements.The basic concept, shown in block diagram in FIG. 10, includeselectroactive strain actuators and sensors bonded to the gantry, alongwith the necessary power, signal, and digital control electronics toachieve vibration reduction. For the purposes of this study, the headwas assumed to be fixed at the end of the gantry. The installation ofactuators was done using a vacuum-bonding procedure.

[0108] “Open loop” testing was then performed. Open loop testinginvolves injecting signals into the actuators and measuring the responseof the gantry to confirm experimentally the analytical modeling doneearlier in this study. This testing was performed with the gantry andhead stationary, as well as moving along some “standard” trajectories.The signal(s) to be passed from the gantry and head motion controller tothe vibration control system were measured as well during these tests.The electoactive actuators were distributed over 10% of the surface areaof the gantry having the maximum strain energy in the first natural modeof vibration. The effectiveness of the actuator distribution at excitingthe first three modes of vibration was modeled using design software.Between 80-84% of all strain energy is in the plate elements; andbetween 62-75% of the plates' energy is extensional strain, andtherefore available for capture by electroactive control devices bondedto the surface. Thus, at least 52% of the strain energy in a mode isavailable. Some of this energy is in the frame/support for the movinghead. As shown in FIG. 13, the extensional strain energy was sorted tomaximize performance for a given amount of electroactive element.

[0109] Damping was added to the structural model. Plots of accelerationversus time at the head, after impact by a hammer, showed roughly 5% ofcritical damping in the first mode with the friction block in place.

[0110] Feedback control was designed using the standard Linear QuadraticRegulator (LQR) approach, ensuring that piezoelectric actuation controlvoltages did not exceed the actuator device limits. Actuation voltagesin the closed feedback loop are proportional to the input disturbanceforces associated with the motion of the gantry. Here, the gantry wasassumed to accelerate in the y-direction (transverse to the gantry axis)at a constant 25 m/S² until maximum velocity of 3 m/s was reached. TheD'Alembert inertial force associated with a 10 kg mass was applied atthe center of gravity of the head. This mass included the 5 kg headmass, plus 5 kg of effective gantry mass.

[0111] The improvements in damping and settling time were thendetermined after simulating the vibration-controlled system's frequencyand time domain responses. Frequency responses are simulated in FIG. 11,measured at a point on the underside of the pick and place head, in they-direction. The reduction in dynamic response to a unit input force isevident in this figure. As shown in FIG. 11, as well as Table I, mode 1closed loop damping was about 12%, mode 2 closed loop damping was about11%, and mode 3 closed loop damping was about 10%. Time responses at thesame point, in the same direction, are simulated in FIG. 12. Thissimulation shows a dramatic reduction in settling time with theelectroactive control. Thus, very effective control can be achieved withvery little additional mass. TABLE I Gantry structural dynamicparameters. Inherent Damping Ratio Frequency Damping Ratio with PiezoMode Description (Hz) (% of critical) Control (%) 1 Twisting about 46 512 gantry axis 2 Bending in xy 93 5 11 (scanning) plane 3 Coupledbend/twist 136  5 10

[0112] The gantry/head structural dynamic properties, from FEM, areshown in Table I. The representative actuator distribution designed herewas 0.5 mm thick, with an area of 330 cm², and a mass of less than 100g. The closed loop modal damping, also shown in Table I, was at leasttwice the assumed 5% value inherent to the gantry with the frictionblock, for all three modes of vibration included in the analysis. Thus,the vibration amplitude and settling time were significantly reduced.

[0113] As shown in FIGS. 14 and 15, the vibration control system inducedchanges in the frequency response and gain control. In this study, thedamping was increased by over one order of magnitude. This increasecorresponds to an increase in placement accuracy of a factor of ten.

[0114] Following the open loop tests, the data was analyzed and thefinal control algorithm design was performed. If necessary, the actuatorand sensor hardware may be modified to ensure compliance with thefunctional requirements. Then, “closed loop” testing of the finalelectroactive vibration control system may be performed. Closed looptesting is generally when actuators are driven at least in part bysignals generated by sensors.

[0115] This study demonstrated that effective active electroactivevibration control of the gantry is possible.

EXAMPLE 2

[0116] A vibration control system in accordance with the invention wasused in a lithography machine. As shown in FIG. 16, which shows thepower spectral density of error signals recorded by a laser metrologysystem, use of the vibration control system resulted in a three-foldreduction in system response in the band from 75 to 125 Hz. Thereduction in the peak using the vibration control system would beexpected to reduce the system image blur by a factor of two-three afterconventional methods are used to reduce peaks at 50 Hz and 225 Hz.Alternatively, in some cases, the vibration control system might be usedto reduce the peaks at 50 Hz or 225 Hz or at other levels. Reducing theimage blur allows the fabrication system to produce finer tracedimensions and feature sizes and improves the accuracy of the featureplacement

[0117] The foregoing description of embodiments and examples of thepresent invention are presented to demonstrate the range ofconstructions to which the invention applies. Those skilled in the artwill appreciate that many other modifications and variations of theinvention as set forth herein above may be made without departing fromthe spirit and scope thereof.

[0118] An additional aspect of the invention discussed herein relates toactively stabilizing (controlling the motion of) wafer stages inlithography tools in six degrees of freedom. FIG. 22,, illustrates anexemplary simplified two-dimensional physics model of a wafer stage base501 and a wafer stage 500. This concept is readily generalizable to areal system in which three-dimensions are of concern.

[0119] The masses of the stage and the base are relatively similar andeach weigh approximately 200 kg. The wafer stage base measuresapproximately 1 m in length × 1 m in width × 0.15 m in thickness. Thewafer stage measures approximately 1.25 m in length × 0.5 m in width ×0.5 m in thickness. Actuator (motor) inputs are represented by thesymbol u_(i), where i represents a specific voice coil motor.Alternative actuators are contemplated which include linear piezoceramicmotors. Sensor outputs are represented by y_(i), where i represents alaser displacement measurement that is collocated with the voice coilinput. Alternative output sensors (including linear variabledisplacement transducers (LVDT's), accelerometers) and sensor locations(nearly co-located, sufficiently colocated) are contemplated for thisexample. Disturbances (represented by d_(i)) to the system includeon-board (including motors, fans, and articulating arms) and off-boarddisturbances (including ground vibrations, air currents, thermalfluctuations).

[0120] The wafer stage 500 is supported on the wafer stage base by apneumatic system 502 comprised of airbearings. This airbearing isprovided to allow the wafer stage 500 to move nearly frictionless withrespect to the wafer base stage 501. The wafer stage base 501 issupported on the ground by a pneumatic system of airmounts 503 and 504.The physical properties of the airmounts are represented by a spring(k1) and a dashpot (c1). The physical properties of the airbearing arerepresented by a spring (k2) and a dashpot (c2). The pneumatic system503 and 504 offsets the weight of the wafer stage 500 and the waferstage base 501 with respect to the ground. The pneumatic system 503 and504 provides a low-frequency (approximately several Hertz) control ofthe plunge and tilt of the wafer stage base 501. Additional highfrequency control of the base stage 501 is provided by the voice coilmotors u1, u2. A microprocessor system 510 is typically used to sensethe outputs and command (actuate) the inputs as a function of thecontrol algorithm implemented by the microprocessor system 510. Thesystem 510 attempts to move or position the wafer stage 500 relative tothe wafer stage base 501 based upon the lithography system requirements.For example the lithography system may require a constant scanningmotion of the wafer stage 500 to be performed during exposure of animage on a wafer. Alternatively, the lithography system may command arapid acceleration of the wafer stage 500 to re-locate the wafer stageto an alternative position. The wafer stage 500 would be required tomake these movements to meet requirements of speed, accuracy, and/orsettling time. Settling time refers to the time required to achieve agiven position within some allowable variation of the absolute position.These prescribed motions with very high accelerations (up to 2 g),create significant reaction forces that are transmitted to the base. Inaddition, these motions cause the compound center of gravity of the basestage system to rapidly change position. Currently base stabilizationcontrol is accomplished by the microprocessor system 510 through theimplementation of six independent single-input, single-output (SISO)controller. Typically a SISO controller is used for each degree offreedom in the specific application. Typical three-dimensional systemscould possess six degrees of freedom for each independently controlledsystem component or stage. SISO controllers are generally susceptible tovariations in the location of the stage relative to the base. This isbecause the individual controller is not provided with additional outputinformation. In one embodiment, the invention described herein, uses amulti-input, multi-output controller (MIMO) to achieve betterperformance than a SISO implementation even with variations of thelocation of the stage relative to the base. In a MIMO implementation thecontrol is accomplished with knowledge of the output and input of morethan one sensor (output) and actuator (input). Additionally, MIMOcontrol architecture allows for the implementation of modern controltechniques, including but not limited to, linear quadratic Gaussian(LQG), H-infinity, and mu synthesis. These techniques cannot beefficiently combined with SISO architecture.

[0121]FIG. 23 illustrates how well a MIMO controller can follow(indicated by ACX roll moment command) a commanded input (stage pitchdisturbance) to the roll moment compared with typical performance of aSISO controller (indicated by Nominal roll moment command). The MIMOcontroller tracks very closely to the disturbance. Thus, it is capableof reacting very quickly to a disturbance force and reject it from thesystem. This improves some combination of the speed, accuracy, orthroughput of the stage.

[0122] In another embodiment of a stage control application in alithography system, it was desired to decrease the settling time for asystem to accurately track a commanded position. FIG. 24 illustrates theblock diagram that describes the embodiment. In this embodiment, threeaccelerometers 600, 601, 602 are used as the sensor for feedbackcontrol. Accelerometers 600 and 601 represent accelerometers thatmeasure x-axis acceleration. Accelerometer 602 represents a singleaccelerometer that measures y-axis acceleration. These measurementswhich are generally proportional to the acceleration are sent to asignal conditioner 606 that buffers the signals and then sends thesignals which are generally proportional to the acceleration of thestage to a single board computer 607. A representative single boardcomputer is Model SBC67 supplied by Innovative Integration Inc. withoffices in Simi Valley, Calif. This processor is a high performancestand-alone digital signal processor single board computer featuringanalog input and output capability. The voltage signals 612 a,b,c arefed into analog inputs. These analog inputs are then converted todigital signals that the processor then applies a control algorithm orfilter to. The algorithm creates a set of digital signals which are thenconverted to analog output signals 613 a,b,c,d. These output signals arethen applied to each of the four motors 608, 609, 610, 611. Motors 608and 609 are x-axis motors that control the position of the stage in thex-axis. Motors 610 and 611 are y-axis motors that control the positionof the stage in the y-axis. X-axis interferometer 603 and Y-axisinterferometer 604 are used to measure the x and y position of the stagerelative to the base of the stage (which is not depicted here forsimplification). The filter (feedback control algorithm) may be designedusing the standard Linear Quadratic Regulator approach, ensuring thatthe motor control signals do not exceed the motor or motor amplifierlimits. Motor control signals in the closed feedback loop areproportional to the accelerometer 600, 601, 602 signals associated withacceleration of the stage. Control design was accomplished by firstcreating a state-space plant model from transfer function data using theSmart ID™ system identification software package commercially availablefrom Active Control Experts, Inc. with offices in Cambridge, Mass. Thefilter (or controller) was then designed through computer simulation andapplication of techniques discussed in Fanson and The Control Handbook,William S. Levine, Editor, CRC Press, 1996.

[0123]FIG. 25 and 26 represent experimental and analytical results ofthe MIMO control applied in this embodiment. The MIMO results arecompared with results in which multiple SISO loops are instead utilized.FIG. 26 illustrates the results when zoomed in between the 0.15 s and0.30 s time period. This figure illustrates that the MIMO controlsettles to within the settle range (approximately 100 on the y-axis ofthe graph) by approximately 0.19 s while the SISO (existing controller)only achieves this performance at approximately 0.26 s. This representsan improvement in settling time of approximately 30%.

What is claimed is:
 1. An motion control system for use with alithography system, said motion control system comprising: a wafer stagebase; at least two actuators for controlling motion; at least twosensors for detecting at least one parameter of displacement of saidwafer base and producing at least two signals in response thereto; andat least one circuit in electrical communication with said actuators andsaid sensors; wherein, upon the detection of said at least one parameterof displacement by said sensors, said sensors signal said circuit,which, in response, activates said actuators to stabilize the waferstage base.
 2. The motion control system of claim 1, said actuators areselected from the group consisting of a voice coil motor andelectroactive stack actuator.
 3. The motion control system of claim 1,said sensors selected from the group consisting of LVDT, accelerometer,laser interferometer, capacitive displacement sensor.
 4. The motioncontrol system of claim 1, said circuit comprising a digital signalprocessor.
 5. The motion control system of claim 1, said circuitcomprising: at least one digital signal processor, at least one analogto digital converter, and at least one digital to analog converter. 6.The motion control system of claim 1, said circuit comprising a controltechnique.
 7. The control technique of claim 6, said control techniqueselected from the group of linear quadratic gaussian, H-infinity, andmu-synthesis.
 8. The motion control system of claim 1, wherein saidactuators stabilize said wafer stage base to closely follow a commandedinput.
 9. An motion control system for use with a lithography system,said motion control system comprising: a wafer stage; at least twoactuators for controlling motion; at least two sensors for detecting atleast one parameter of displacement of said wafer base and producing atleast two signals in response thereto; a signal conditioner; and asingle board computer wherein, upon the detection of said at least oneparameter of displacement by said sensors, said sensors feed a signal tosaid signal conditioner, said signal conditioner feeds a signal to saidsingle board computer, and said single board computer commands saidactuators to command said wafer stage to track a commanded position. 10.The motion control system of claim 9, wherein said actuators areselected from the group consisting of voice coil motor and electroactivestack actuator.
 11. The motion control system of claim 9, wherein saidsensors are selected from the group consisting of LVDT, accelerometer,laser interferometer, capacitive displacement sensor.
 12. The motioncontrol system of claim 9, wherein said wafer stage is commanded totrack a commanded position within 0.19 seconds.